The use of cryogens, particularly carbon dioxide and nitrogen, has been investigated for a number of years for such in-transit refrigeration needs. Some of these systems have sprayed the cold vapor produced by the expansion of the liquefied gas directly into the product storage region, and examples of such systems are shown in U.S. Pat. Nos. 3,287,925, 3,446,028, 3,525,235 and 3,638,443. Other systems, such as those shown in U.S. Pat. Nos. 3,757,531 and 4,045,972, cause the cryogens to vaporize within a heat-exchanger located within the insulated storage chamber and then employ the expanding gases evolved from the evaporating liquid to drive an air-motor connected to a fan blade which directs air from the storage chamber past a heat-exchange coil. A somewhat related system utilizing the direct-injection of cryogen into an insulated storage chamber was embodied in the so-called "clip-on" units for containerized shipment exemplified by U.S. Pat. Nos. 4,033,140, 4,038,833 and 4,048,813.
A problem shared in common by these systems was that the arrangement had to be such that the air could circulate around and envelop the refrigerated load, otherwise the portion of the product which rested directly against the floor or against the walls would tend to become warm despite the fact that air at the top of the storage chamber was maintained within the desired temperature limits.
There were also some containers which were developed for shipping and/or storing perishable foods which relied upon the supply of a quantity of solid CO.sub.2 to an upper chamber, and exemplary of such units are U.S. Pat. Nos. 3,800,554, 3,864,936 and 4,299,429. U.S. Pat. No. 3,561,226 extends this type of system to a delivery truck and employs a pair of elongated compartments which are filled with packed CO.sub.2 snow by expanding liquid CO.sub.2 through orifices at each end of these compartments. Still other patents show devices wherein liquid CO.sub.2 or snow is employed to condense a refrigerant, and the condensed refrigerant is then allowed to enter an evaporator through a thermostatically controlled valve to supply the cooling needs within a storage chamber. Exemplary of these systems are U.S. Pat. Nos. 1,943,994, 1,959,994 and 3,225,558.
Still another attempt to overcome the basic problem of getting the cooling to the load is exemplified by U.S. Pat. No. 4,129,432 wherein a plurality of carbon dioxide containers are maintained, usually just below the roof of an insulated storage chamber; the pressure of the liquid CO.sub.2 is controlled to maintain the uninsulated containers at a desired low temperature by venting CO.sub.2 vapor as necessary. The cold, more dense atmosphere created adjacent the uninsulated containers is permitted to gravitate downward through passageways along the walls created by vertical ribs. Still another system of this type is shown in U.S. Pat. No. 3,308,630 wherein an insulated enclosure is provided which is cooled with a liquid cryogen, such as liquid nitrogen or liquid CO.sub.2. The insulated enclosure is built with an elaborate system of ducts created in the walls, floor and ceiling at a region just adjacent the interior skin and thus separated from the outer skin by the thermal insulation material. The vaporizers are located in upper locations, and the cold vapor which is created, as well as the cooling of the compartment atmosphere adjacent the ceiling, causes cold fluid flow downward. The special structural requirements of the floor of such a system and the loss in storage space from the creation of the passageway network are considered to be substantial drawbacks to the commercialization of such a system. Accordingly, the search has continued for more effective cryogenic refrigeration systems for transports.